A hard disk drive (HDD) is a data storage device used for storing and retrieving digital information using rapidly rotating discs coated with magnetic material. A typical HDD includes one or more rigid rapidly rotating disks with magnetic heads arranged on a moving actuator arm to read and write data onto the disk surfaces. The disks are placed about a spindle, and are spun rapidly about the spindle as information is written to and read from the disk surface as it rotates past magnetic read-and-write head or heads that operate closely over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material under it. The actuator arm is also known as a slider, as it floats or slides over the surface of the disk, carrying the head at a correct distance from the recording medium for reading and writing. As such, the read-and-write head can also be known as a slider head.
In order to accurately control the read-and-write (or read/write) head, a servo system is provided for control of the HDD, which can be for generating position feedback signals. HDD servo systems typically involve three kinds of control tasks: track-seeking control, track-following control, and setting control. In track-seeking control, the head positioning servomechanism moves the read/write head as fast as possible from one track to another. Once the head reaches the target track, it is regulated over the track so that the head can follow the track as precisely as possible during the operation of reading or writing data, in track-following control. In settling control, the servo provides for smooth settling, i.e. transition between the track-seeking and track-following modes without any impact.
In a conventional dedicated servo system, servo information is provided on a layer distinct from the data recording layer. Typically, the servo layer is provided on a separate layer from a disk including a data recording layer. In an embedded servo system, servo information is interspersed with data across the entire surface of all of the hard disk platter surfaces. The servo information and data are read by the same read/write heads, where the heads do not need to wait for the disk to rotate the servo information into place.
Recent developments have also introduced a buried dedicated servo system where servo information is provided on a layer distinct from the data recording later. Additionally, a servo recording layer is provided as a layer secondary to the data recording layer, and where both servo layer and data recording layer are provided on a single disk. Typically, the servo later is provided below the data layer on the disk. The servo layer includes a servo pattern which can be used in position error signal (PES) demodulation, which provides position error feedback to the servo control system to position the read/write head in moving across the surface of a rotating disk.
Buried servo systems can provide continuous PES analysis and thus can improve servo performance in HDD operations. However, the quality of the determined PES may be degraded due to the coupling of data signals to the servo channel. Simulation carried out by the present applicant shows that the coupling can contribute to the repeatable run out (RRO) component (harmonics of the frequency of spindle speed) in the PES demodulation noise.
As such, although a buried dedicated servo system supports high track density in the development of present day HDD technology, a concern is raised where data layer coupling interference may cause irregularities in the identification of the read/write head positioning.